Rapamycin is a macrolide antibiotic produced by Streptomyces hygroscopicus which was discovered first for its properties as an antifungal agent. It adversely affects the growth of fungi such as Candida albicans and Microsporum gypseum. Rapamycin, its preparation and its antibiotic activity were described in U.S. Pat. No. 3,929,992, issued Dec. 30, 1975 to Surendra Sehgal et al. In 1977 Martel, R. R. et al. reported on immunosuppressive properties of rapamycin against experimental allergic encephalitis and adjuvant arthritis in the Canadian Journal of Physiological Pharmacology, 55, 48-51 (1977). In 1989, Calne, R. Y. et al. in Lancet, 1989, no. 2, p. 227 and Morris, R. E. and Meiser, B. M. in Medicinal Science Research, 1989, No. 17, P. 609-10, separately reported on the effectiveness of rapamycin in inhibiting rejection in vivo in allograft transplantation. Numerous articles have followed describing the immunosuppressive and rejection inhibiting properties of rapamycin, and clinical investigation has begun for the use of rapamycin in inhibiting rejection in transplantation in man.
Rapamycin is insoluble in water and is only slightly soluble in solubilizers, such as propylene glycol, glycerin and PEG 400, commonly used in preparing parenteral formulations. It is only sparingly soluble in PEG 300 and is insoluble or very slightly soluble in commonly used aqueous injectable co-solvent systems, such as, 20% ethanol/water, 10% DMA/water, 20% Cremophor.RTM. EL/water and 20% polysorbate 80/water. For these reasons clinically and commercially acceptable injectable formulations of rapamycin have been difficult to make. An injectable composition of rapamycin is described in European Patent Publication No. 0041795, published Dec. 16, 1981. In this injectable formulation rapamycin is first dissolved in a low boiling point organic solvent, namely, acetone, methanol or ethanol. This solution is then mixed with a nonionic surfactant selected from polyoxyethylated fatty acids; polyoxyethylated fatty alcohols; and polyoxyethylated glycerin hydroxy fatty acid esters, e.g. polyoxyethylated castor oil, exemplified by Cremophor.RTM. EL and polyoxyethylated hydrogenated castor oil, exemplified by Cremophor.RTM. RH 40 and Cremophor.RTM. RH 60. Cremophor.RTM. EL is the primary nonionic surfactant used in the examples.
The primary immunosuppressive agent presently used for inhibiting rejection in the allograft transplantation of organs in man is cyclosporine (Sandimmune.RTM.). Cyclosporine is a cyclic polypeptide consisting of 11 amino acids. The intravenous injectable formulation of Sandimmune.RTM. (IV) is a sterile ampul containing, per ml, 50 mg of cyclosporine, 650 mg of Cremophor.RTM. EL and alcohol Ph Helv. (32.9% by volume) (under nitrogen). For administration this mixture is diluted further with 0.9% Sodium Chloride Injection or 5% Dextrose Injection before use. (Physicians' Desk Reference, 45th ed., 1991, pp. 1962-64, Medical Economics Company, Inc.) The macrolide molecule designated FK506, which has certain structural similarities to rapamycin, is also currently undergoing clinical investigation for inhibiting rejection in allograft organ transplantation in man. FK506 is isolated from Streptomyces tsuskubaensis and is described in U.S. Pat. No. 4,894,366 to Okuhara et al., issued Jan. 16, 1990 R. Venkataramanan et al., in Transplantation Proceedings, 22, No. 1, Suppl., 1 pp 52-56 (February 1990), report that the intravenous injectable formulation of FK506 is provided as a 10 mg/ml solution of FK506 in polyoxyethylated castor oil (HCO-60, a surfactant) and alcohol. The intravenous preparation must be diluted with saline or dextrose and administered as an infusion for 1 to 2 hours.